It is known that silica-filled rubber vulcanizates differ clearly in their crosslinking behavior from rubber compounds filled with carbon black. Complete vulcanization is significantly delayed and the crosslink density is too low for practical application. Accordingly, an activator, for example a mercaptopropyl silyl ether or bis-(trialkoxysilylpropyl)-tetrasulfide, generally has to be added. By contrast, mercaptopropyl silyl ethers have the unpleasant property of considerably shortening the processing time of the rubber compounds under processing conditions, so that they can only be used in special cases where a long processing time is not a problem (industrial rubber particles, not tires), cf. the comparison tests in DE 2 255 577. By contrast, bis-(tri-alkoxysilylpropyl)-tetrasulfide (DE 2 255 577) has the disadvantage that complete vulcanization lasts a very long time so that long production times are inevitable (see the Comparison Example of the present application). In this case, combination with a compound corresponding to formula (II) leads only to an unwanted reduction in the scorch time whereas complete vulcanization is actually extended.
EP 447 066 describes special silica-filled rubber compounds containing inter alia a special silane. There are no references in this document either to the vulcanization kinetics or to the production of the silane. According to our own tests, the silanes disclosed therein shorten the scorch time and, accordingly, are industrially unsuitable for a number of applications where processing safety is an important requirement.
DE 42 33 197 describes certain benzthiazole derivatives as vulcanization accelerators which are more effective than conventional vulcanization accelerators in silica-filled rubbers, but which do not produce any significant improvement in modulus or abrasion resistance in tires.
Accordingly, there is still a need for rubber compounds, more particularly silica-filled rubber compounds, which show good vulcanization kinetics and which, after vulcanization, give rubber articles, more particularly tires, with inter alia a good modulus value and high abrasion resistance.